Complex transcriptional regulation of myc family gene expression in the developing mouse brain and liver

Spotlight on New Therapeutic Opportunities for MYC-Driven Cancers

MYC can be considered to be one of the most pressing and important targets for the development of novel anti-cancer therapies. This is due to its frequent dysregulation in tumors and due to the wide-ranging impact this dysregulation has on gene expression and cellular behavior. As a result, there have been numerous attempts to target MYC over the last few decades, both directly and indirectly, with mixed results. This article reviews the biology of MYC in the context of cancers and drug development. It discusses strategies aimed at targeting MYC directly, including those aimed at reducing its expression and blocking its function. In addition, the impact of MYC dysregulation on cellular biology is outlined, and how understanding this can underpin the development of approaches aimed at molecules and pathways regulated by MYC. In particular, the review focuses on the role that MYC plays in the regulation of metabolism, and the therapeutic avenues offered by inhibiting the metabolic pathways that are essential for the survival of MYC-transformed cells.

Targeting Myc Interacting Proteins as a Winding Path in Cancer Therapy

MYC, as a well-known oncogene, plays essential roles in promoting tumor occurrence, development, invasion and metastasis in many kinds of solid tumors and hematologic neoplasms. In tumors, the low expression and the short half-life of Myc are reversed, cause tumorigenesis. And proteins that directly interact with different Myc domains have exerted a significant impact in the process of Myc-driven carcinogenesis. Apart from affecting the transcription of Myc target genes, Myc interaction proteins also regulate the stability of Myc through acetylation, methylation, phosphorylation and other post-translational modifications, as well as competitive combination with Myc. In this review, we summarize a series of Myc interacting proteins and recent advances in the related inhibitors, hoping that can provide new opportunities for Myc-driven cancer treatment.

Chromosomal location targets different MYC family gene members for oncogenic translocations

The MYC family of cellular oncogenes includes c-Myc, N-myc, and L-myc, which encode transcriptional regulators involved in the control of cell proliferation and death. Accordingly, these genes become aberrantly activated and expressed in specific types of cancers. For example, c-Myc translocations occur frequently in human B lymphoid tumors, while N-myc gene amplification is frequent in human neuroblastomas. The observed association between aberrations in particular MYC family genes and specific subsets of malignancies might reflect, at least in part, tissue-specific differences in expression or function of a given MYC gene. Since c-Myc and N-myc share substantial functional redundancy, another factor that could influence tumor-specific gene activation would be mechanisms that target aberrations (e.g., translocations) in a given MYC gene in a particular tumor progenitor cell type. We have previously shown that mice deficient for the DNA Ligase4 (Lig4) nonhomologous DNA end-joining factor and the p53 tumor suppressor routinely develop progenitor (pro)-B cell lymphomas that harbor translocations leading to c-Myc amplification. Here, we report that a modified allele in which the c-Myc coding sequence is replaced by N-myc coding sequence (NCR allele) competes well with the wild-type c-Myc allele as a target for oncogenic translocations and amplifications in the Lig4/p53-deficient pro-B cell lymphoma model. Tumor onset, type, and cytological aberrations are similar in tumors harboring either the wild-type c-Myc gene or the NCR allele. Our results support the notion that particular features of the c-Myc locus select it as a preferential translocation/amplification target, compared to the endogenous N-myc locus, in Lig4/p53-deficient pro-B cell lymphomas.

MYC oncogenes and human neoplastic disease

c-myc, N-myc and L-myc are the three members of the myc oncoprotein family whose role in the pathogenesis of many human neoplastic diseases has received wide empirical support. In this review, we first summarize data, derived mainly from non-clinical studies, indicating that these oncoproteins actually serve quite different roles in vivo. This concept necessarily lies at the heart of the basis for the observation that the deregulated expression of each MYC gene is reproducibly associated with only certain naturally occurring malignancies in humans and that these genes are not interchangeable with respect to their aberrant functional consequences. We also review evidence implicating each of the above MYC genes in specific neoplastic diseases and have attempted to identify unresolved questions which deserve further basic or clinical investigation. We have made every attempt to review those diseases for which significant and confirmatory evidence, based on studies with primary tumor material, exists to implicate MYC members in their causation and/or progression.

A novel intron element operates posttranscriptionally To regulate human N-myc expression

Precisely regulated expression of oncogenes and tumor suppressor genes is essential for normal development, and deregulated expression can lead to cancer. The human N-myc gene normally is expressed in only a subset of fetal epithelial tissues, and its expression is extinguished in all adult tissues except transiently in pre-B lymphocytes. The N-myc gene is overexpressed due to genomic amplification in the childhood tumor neuroblastoma. In previous work to investigate mechanisms of regulation of human N-myc gene expression, we observed that N-myc promoter-chloramphemicol acelyltransferase reporter constructs containing sequences 5' to exon 1 were active in all cell types examined, regardless of whether endogenous N-myc RNA was detected. In contrast, inclusion of the first exon and a portion of the first intron allowed expression only in those cell types with detectable endogenous N-myc transcripts. We investigated further the mechanisms by which this tissue-specific control of N-myc expression is achieved. Using nuclear run-on analyses, we determined that the N-myc gene is actively transcribed in all cell types examined, indicating a posttranscriptional mode of regulation. Using a series of N-myc intron 1 deletion constructs, we localized a 116-bp element (tissue-specific element [TSE]) within the first intron that directs tissue-specific N-myc expression. The TSE can function independently to regulate expression of a heterologous promoter-reporter minigene in a cell-specific pattern that mirrors the expression pattern of the endogenous N-myc gene. Surprisingly, the TSE can function in both sense and antisense orientations to regulate gene expression. Our data indicate that the human N-myc TSE functions through a posttranscriptional mechanism to regulate N-myc expression.

Differential regulation of the N-myc proto-oncogene by ROR alpha and RVR, two orphan members of the superfamily of nuclear hormone receptors

ROR alpha1 and RVR are orphan members of the superfamily of nuclear hormone receptors which constitutively activate and repress, respectively, gene transcription by binding to a common DNA sequence. In an attempt to understand the physiological functions of these two transcription factors, we aimed to identify target genes. We have identified a consensus binding site for ROR alpha1 and RVR in the first intron of the N-myc gene that we designated N-myc RORE (ROR response element). Unlike most of the intronic sequence, the region encompassing the N-myc RORE is highly conserved between human and mouse, underscoring its importance. Our studies revealed that ROR alpha1 and RVR specifically bind to the human and mouse N-myc ROREs and transactivate and transrepress, respectively, reporter constructs containing the ROREs. Moreover, Northern blot analysis demonstrated a direct modulation of an exogenously introduced N-myc gene by ROR alpha1 and RVR in COS-1 cells. This effect is mediated through the N-myc RORE, since mutation of this site abolished the regulatory effects of both receptors. While transfection of ROR alpha1 in P19 embryonic carcinoma cells had no effect on the levels of endogenous N-myc mRNA, RVR down-regulated its expression. The regulatory function of the N-myc RORE was further demonstrated by the rat embryonic fibroblast (REF) transformation assay. Mutation of the RORE increased the oncogenic potential of the N-myc gene in the REF assay. The foci were more numerous and significantly larger with the mutated than with the wild-type N-myc gene, regardless of ROR alpha1 or RVR expression. Moreover, concomitant expression of ROR alpha1 and wild-type N-myc resulted in a twofold increase in the number of transformed foci. In contrast, RVR expression resulted in the formation of foci that could be established as permanent clones with a very low frequency compared to foci transformed in its absence. These observations show that ablation of the RORE results in a more oncogenic form of N-myc and suggest that deregulation of the activity of the ROR alpha1 and RVR could contribute to the initiation and progression of certain neoplasias.

Mice develop normally without the H1(0) linker histone

H1 histones bind to the linker DNA between nucleosome core particles and facilitate the folding of chromatin into a 30-nm fiber. Mice contain at least seven nonallelic subtypes of H1, including the somatic variants H1a through H1e, the testis-specific variant H1t, and the replacement linker histone H1(0). H1(0) accumulates in terminally differentiating cells from many lineages, at about the time when the cells cease dividing. To investigate the role of H1(0) in development, we have disrupted the single-copy H1(0) gene by homologous recombination in mouse embryonic stem cells. Mice homozygous for the mutation and completely lacking H1(0) mRNA and protein grew and reproduced normally and exhibited no anatomic or histologic abnormalities. Examination of tissues in which H1(0) is normally present at high levels also failed to reveal any abnormality in cell division patterns. Chromatin from H1(0)-deficient animals showed no significant change in the relative proportions of the other H1 subtypes or in the stoichiometry between linker histones and nucleosomes, suggesting that the other H1 histones can compensate for the deficiency in H1(0) by occupying sites that normally contain H1(0). Our results indicate that despite the unique properties and expression pattern of H1(0), its function is dispensable for normal mouse development.

Suppression of Myc, but not E1a, transformation activity by Max-associated proteins, Mad and Mxi1

Mad and Mxi1, two members of the Myc-related basic-region helix-loop-helix/leucine-zipper family of proteins, associate directly with Max to form sequence-specific DNA binding heterodimers that are transactivation-incompetent. Mad-Max complexes have been shown to exert a strong repressive effect on Myc-induced transactivation, perhaps through the competitive occupation of common promoter binding sites also recognized by active Myc-Max heterodimers. To place these recent biochemical observations in a biological context, mad and mxi1 expression vectors were tested for their ability to influence Myc transformation activity in the rat embryo fibroblast cooperation assay. Addition of an equimolar amount of mad or mxi1 expression vector to mouse c-myc/ras cotransfections resulted in a dramatic reduction in both the number of foci generated and the severity of the malignant phenotype. Myc-specific suppression by Mad and Mxi1 was demonstrated by their ability to affect c- and N-myc-, but not ela-, induced transformation. In contrast, mad and mxi1 expression constructs bearing deletions in the basic region exerted only mild repressive effects on Myc transformation activity, suggesting that occupation of common DNA binding sites by transactivation-incompetent Mad-Max or Mxi1-Max complexes appears to play a more dominant role in this suppression than titration of limited intracellular pools of Max away from active Myc-Max complexes. Thus, these biological data support a current model for regulation of Myc function in which relative intracellular levels of Mad and Mxi1 in comparison to those of Myc may determine the degree of activation of Myc-responsive growth pathways.

Expression of the woodchuck N-myc2 retroposon in brain and in liver tumors is driven by a cryptic N-myc promoter

The woodchuck intronless proto-oncogene N-myc2 was initially discovered as a frequent target site for hepadnavirus integration in hepatocellular carcinoma. N-myc2 possesses characteristics of a functional retroposon derived from the woodchuck N-myc gene. We have investigated the regulatory signals governing N-myc2 expression and found that a short promoter, including a variant TATA box and potential binding sites for several transcription factors, is localized in the N-myc2 sequences homologous to the 5' untranslated region of the second N-myc exon. The corresponding region in the intron-containing woodchuck N-myc gene also exhibited promoter activity in transient transfection assays. The high evolutionary conservation of these sequences in mammalian N-myc genes suggests that they contain a cryptic N-myc promoter which may be unmasked in the particular context provided by the N-myc2 retroposon. Although N-myc2, like the woodchuck N-myc gene, contributes to an extended CpG island and was found constitutively hypomethylated, it presents a highly restricted expression pattern in adult animals. Whereas the intron-containing N-myc gene is expressed at low levels in different tissues, N-myc2 mRNA was detected only in brain tissue, raising questions about the functional significance of the maintenance of a second N-myc gene in the woodchuck genome.

Expression of the woodchuck N-myc2 retroposon in brain and in liver tumors is driven by a cryptic N-myc promoter

The woodchuck intronless proto-oncogene N-myc2 was initially discovered as a frequent target site for hepadnavirus integration in hepatocellular carcinoma. N-myc2 possesses characteristics of a functional retroposon derived from the woodchuck N-myc gene. We have investigated the regulatory signals governing N-myc2 expression and found that a short promoter, including a variant TATA box and potential binding sites for several transcription factors, is localized in the N-myc2 sequences homologous to the 5' untranslated region of the second N-myc exon. The corresponding region in the intron-containing woodchuck N-myc gene also exhibited promoter activity in transient transfection assays. The high evolutionary conservation of these sequences in mammalian N-myc genes suggests that they contain a cryptic N-myc promoter which may be unmasked in the particular context provided by the N-myc2 retroposon. Although N-myc2, like the woodchuck N-myc gene, contributes to an extended CpG island and was found constitutively hypomethylated, it presents a highly restricted expression pattern in adult animals. Whereas the intron-containing N-myc gene is expressed at low levels in different tissues, N-myc2 mRNA was detected only in brain tissue, raising questions about the functional significance of the maintenance of a second N-myc gene in the woodchuck genome.